Prime Minister David Cameron has said that porn blockers should be turned on by default on computers used in houses with children.

Writing in the Daily Mail, Cameron said that while universal 'default on' filters were not practical - and were rejected by his ministers earlier this week - he would now attempt to force ISPs to ask if new users have children in the house before listing options for filtered content.

If parents answer 'yes' they will be asked to set up filters blocking sexual content, individual sites, turning off access at certain times and other measured designed to protect children from the darker corners of the net.

ISPs would be asked to check that those setting up the filters are over 18.

"To me, the fact that so many children have visited the darkest corners of the internet is not just a matter of concern - it is utterly appalling," Cameron wrote.

"A silent attack on innocence is underway in our country today and I am determined that we fight it with all we've got."

Cameron added that

"Let me reiterate the key points: with our new system, every parent will be prompted to protect their child online. If they don't make choices, protection will be automatically on. No other Government has taken such radical steps before. And once all this is in place, Britain will have the most robust internet child protection measures of any country in the world - bar none."

The Daily Mail and other tabloid newspapers have been campaigning for years to have filters for porn and other content imposed on internet users.